Carbonitriding and other thermal treatment of columbium steels

ABSTRACT

FERRITIC GRAIN COARSENING, AND CONCOMITANT LOSS OF STRENGTH AND FATIGUE PROPERTIES, ARE AVOIDED IN CARBONITRIDING AND OTHER PROCESSES IN WHICH COLD-FORMED STEEL ARTICLES ARE HEATED ABOVE THE STRAIN RECRYSTALLIZATION TEMPERATURE BUT BELOW THE A3 CRITICAL TEMPERATURE. USE OF STEEL CONTAINING ABOUT 0.006-0.018% COLUMBIUM FOR COLDFORMED PARTS HEATED TO CARBONITRIDING TEMPERATURES PRODUCES AN ASTM GRAIN SIZE OF 5 OR FINER.

United States Patent O 3,673,008 CARBONITRIDING AND OTHER THERMAL TREATMENT OF COLUMBIUM STEELS Mahlon E. Wood, Farmington, Mich., assignor to National Steel Corporation No Drawing. Filed Apr. 25, 1969, Ser. No. 819,424 Int. Cl. C21d 1/74, 7/00 US. Cl. 148-12 12 Claims ABSTRACT OF THE DISCLOSURE Ferritic grain coarsening, and concomitant loss of strength and fatigue properties, are avoided in carbonitriding and other processes in which cold-formed steel articles are heated above the strain recrystallization temperature but below the A critical temperature. Use of steel containing about 0.006-0.018% columbium for coldformed parts heated to carbonitriding temperatures produces an ASTM grain size of 5 or finer.

BACKGROUND OF THE INVENTION Many cold-formed steel articles, e.g. small automotive parts, are carbonitrided. As a result of the cold working, strain recrystallization followed by grain coarsening (growth of recrystallized ferrite grains) occurs in the steel upon heating to carbonitriding temperatures, because the carbonitriding temperatures are above the strain recrystallization temperature of the steel. Also, in regions of the article which have been strained within a critical range by the cold working, heating may cause one or more ferrite grains to blow, i.e., undergo exaggerated, abnormal growth, without passing through any real stage of recrystallization. Such types of ferritic grain coarsening are direct results of cold working.

The strain recrystallization which leads to ferrite grain coarsening differs from the allotropic recrystallization which occurs when ferrite is transformed to austenite. Most carbonitriding is eflected at a temperature above the A critical temperature but below the A critical temperature of the steel, so allotropic recrystallization is started but not complete. The ferrite grains which are not transformed to austenite at the carbonitriding temperature are subject to the coarsening which results from cold work. Hence, ferritic grain coarsening is to be distinguished from the austenite grain coarsening which occurs above the A temperature. Grain coarsening in the austenite region is not a result of cold work, the original ferritic microstructure having been eliminated by the allotropic recrystallization to austenite which is completed at the A temperature. Rather, the coarsening of the austenite grains is a result of the high temperature imparting to the'grains of austenite sufiicient energy for the grains to grow to an same temperatures as carbonitriding. Accordingly, mainobjects of the invention are the provision of cold-formed steel articles having fine-grained microstructures, and methods for producing such articles.

DETAILED DESCRIPTION In accordance with a preferred embodiment of, the

invention, low-cost, low-carbon steels containing extremely small amounts of columbium are cold-formed and carbonitrided without undergoing objectionable ferrite grain coarsening. Steels used in accordance with the invention consist of about 0.006-0.018% columbium, about 0.05- 0.15% carbon, about 0.250.75% manganese, about 0.03- 0.11% silicon, and the balance essentially iron. The term essentially iron as used herein is intended to include impurities and residual elements which are commonly found in steels. All composition percentages specified herein are by weight. As indicated by the silicon contents, the steels are semi-killed.

Steels consisting of about 0.0080.015% columbium, 0.080.13% carbon, about 0.300.60% manganese, about 0.04'0.08 %silicon and the balance essentially iron, are preferred. It has been found that steels consisting of about 0.01% columbium, about 0.11% carbon, about 0.50% manganese, about 0.06% silicon and the balance essentially iron, are particularly desirable in carrying out the invention.

The steels are produced by standard metallurgical practice for making fine-grained steels. In producing the steels, a conventional carbon steel melt composition is adjusted to provide the desired columbium content by addition of columbium, in ferrocolumbium or in other form, to the ladle or mold. The steels are hot rolled using standard steelmaking practices promoting fine grain size.

Steel stock with the above chemistry is cold-formed into desired shape for articles desired to have a hard, wear-resistant surface and a strong, tough core. An illustrative but by no means exhaustive list of fabricated automotive parts suitable for application of inventive principles includes bearing retainers, front end suspension ball-joint sockets, and striker plates in door latches. The steels have excellent formability, so articles of widely differing shapes can be made in accordance with the invention and are not limited to automotive applications.

The cold worked article is heated in a carbonitriding Carbonitriding media are usually gaseous, but can be liquid. One gaseous carbonitriding atmosphere which can be employed is a mixture of vaporized hydrocarbon (as a source of carbon), ammonia (as a source of nitrogen), and a carrier gas such as endothermic generator gas. Carbonitriding media usable in accordance with the invention can vary widely in composition while being effective to introduce carbon and nitrogen into the surface of the heated article. Carbonitriding media are discussed at length in Volume 2 of the ASM Handbook (8th Edition) published in- 1964 by The American Society for Metals, Metals Park, Ohio, pages 119-132, the disclosure of which is hereby incorporated by reference.

The article is heated in the carbonitriding medium for a time to impart a hard, wear-resistant case on the article. Carbonitrided cases are generally from about 0.001"- 0.030" deep, depending upon the intended service application of the article. The time required to produce a case of any given depth depends upon the carbonitriding medium and upon the heating temperature.

The heating temperatures at which carbonitriding is effected are above the strain recrystallization temperature of steel, which latter is about 842 F. Strain recrystallization and grain coarsening proceed slowly at this temperature level, but increase in rate with increasing temperatures and proceed relatively rapidly at temperatures of about 1150 F. and above. Since most carbonitriding is effected at temperatures above the A; critical tempera- 3 1 ture of the steel (about 1333 F.) to achieve fast penetration, for safety reasons and other purposes, the rate of recrystallization and coarsening of recrystallized ferrite grains proceeds rapidly at carbomtriding temperatures. Moreover, differently shaped regions of the cold-formed part are subjected to different amounts of strain by the cold working, and any region strained within a critical range of about 7-15 in extension or compression heretofore presented the danger that abnormal, exaggerated growth of ferrite grains would occur in this region. Grains tend to blow in the region of the bend area when steel is bent through an angle of about 90, since in the bend area will be a region which has been subjected to strain within the critical range as well as regions which have been strained more and less than the critical amount. However, it will be appreciated that critically strained regions can be produced in steel bent through angles of more and less than 90 in forming steel articles, depending upon the shape of the article.

Since ferritic grain coarsening uniquely occurs below the A, critical temperature of the steel (about 1580"- 1640 F., depending upon the carbon content, for steels used in accordance with the invention) and since most carbonitriding is effected at temperatures below the A temperature for maximum effectiveness of ammonia, to avoid departure from dimensional tolerances, or for other reasons, carbonitriders heretofore have been severely handicapped by ferritic grain coarsening. However, it has been found that in operation in accordance 'with the invention, the coarsening of the recrystallized ferrite grains is minimized, and the tendency for ferrite grains to blow in critically strained regions is reduced, so that parts carbonitrided in accordance with the invention have an ASTM grain size of 5 or finer. Such grain size is very acceptable, and the loss of strength and fatigue properties heretofore associated with carbonitriding is avoided.

Carbonitriding is most commonly eflected at temperatures of about 1375 1550 F. for the steels herein involved. The heated part is conventionally quenched in water, oil or gas, depending upon allowable distortion and other factors, and any of these cooling practices can be used in accordance with the invention. Parts carbonitrided in accordance with the invention exhibit good core ductility, and can be made in any desired thickness. Since the steels employed are cheaper than aluminum-killed steels previously used for some applications, but can be substituted therefor, economic advantages accrue from practice of the invention in this regard.

In operations where a cold-formed part must be heated to a temperature above the strain recrystallization temperature but below the A critical temperature, such as intercritical annealing (effected at a temperature above the A; critical temperature but below the A critical temperature), but is not desired to be carbonitrided, the heat ing is effected in an inert or other suitable heat-treating atmosphere which is free of constituents which enter and harden the surface of the article. Further, the heated parts are allowed to cool relatively slowly, e.g. in the furnace or other atmosphere, instead of being cooled by quenching. Otherwise, the procedure is similar to that described for carbonitriding. The ferritic grain coarsening from the cold working, which would normally be expected to occur at the elevated temperatures, is retarded as in the case where the article is heated. in a carbonitriding medium.

The present invention is further illustrated by the following specific example which is not to be taken as limiting the principles of the'invention as defined by the appended claims.

Example Sheet steel stock 0.150" in thickness, consisting of 0.014% columbium, 0.11% carbon, 0.54% manganese, 0.05% silicon and the balance essentially iron (including 0.008% phosphorus, 0.016% sulfur and 0.06% copper), and having a grain size of 8 and finer, is cold-formed into a part for an automotive front suspension. The part is generally L-shaped in cross-section, including a bend of about The part'is'carbonitrided for 2 hours in an atmosphere of 4-5% natural gas, 5-6% ammonia and balance an endothermic carrier gas in a continuous furnace having zones of 1400 F., 1450' F. and 1510 F., with approximately equal time in each zone, to obtain a case depth of 0.0080.014". The part is quenched from 1510 F. in oil at F. The carbonitrided steel part has a grain size of 7-8 and finer. This is in marked contrast to a part made from a columbium-free, low-carbon rimmed drawing quality steel which was used for the part and which, in identical treatment, underwent severe-grain growth and had a grain. size of 4'-5 with-a few scattered size 3 grains.

I claim: 1. Process for making a steel article having a finegrained microstructure, comprisingproviding steel stock consisting of by weight about '0.0060.018% columbium, about ODS-0.15% carbon, about 0.25-0.75 manganese, about 0.03 0.11% silicon, and the balance essentially iron, cold working the steel stock to form an article, heating the cold-wonked steel article to a temperature above the strain recrystallization temperature of the steel but below the A critical temperature of the steel, and cooling the heated article. 2. The process of claim 1, the cold worked steel article being. heated. in a carbonitriding medium to case-harden the article. 3. The process of claim 1, I the steel stock consisting of by weight about 0.008-

0.015% columbium, about 0.08 0.13% carbon, about 0.30-0.'60% manganese, about 0.04-0.08% silicon, and the balance essentially iron. p 4. The process of claim 1, the steel stock consisting of by weight about 0.01% columbium, about 0.11% carbon, about I 0.50% manganese, about-0.06% silicon, and the balance essentially iron. 5. The process of claim 1, theiin cooled article having an AS'I M grain size of 5' or er. 6. The process of claim 1, at least a portion of the steel stock being strained by the cold working to within a range of about 7-l5%. 7. As anew use of steel stock consisting of by weight about 0.006-0.018% columbium, about 0.05-0.15% carbon, about 0.25-0.75% manganese, about ODS-0.11% silicon, and the balance essentially iron, the process comprising cold working the steel stock to form an article, heating the cold worked steel article to a temperature above the strain recrystallization temperature of the steel and above the A critical temperature of the steel, but below the A; critical temperature of the steel, and 1 cooling the heated article. 8. The new use of claim 7, 1 the cold worked steel article being heated in a carbonitriding medium to case-harden the article. 9. The new use of claim 7, the steel consisting of by weight about 0.008-0.0 l5% columbium, about 0 .08-0.l3% carbon, about 0.30- 0.60% manganese, about 0.040.08% silicon, and the balance essentially iron. 10. The new use of claim 7, the steel stock consisting by weight of about 0.01% columbium, about 0.11% carbon, about 0.50% manganese, about 0.06% 'silicon, and the balance essentially iron. I 11. The new use of claim 7, the cooled article having an ASTM grain size of 5 or finer. Y I

5 12. The new use of claim 7, at least a portion of the steel stock being strained by 6 FOREIGN PATENTS 1,101,193 1/1968 Great Britain.

the cold working to within a range of about 7-15 OTHER REFERENCES References Cited 5 Boron, Calcium, Columbium and Zirconium in Iron and UNITED STATES PATENTS Steel; pp. 142-147, Alloys of Iron Research, Monograph 7/1970 Shimizu et a1. Senes, 1957; Grange et al.; W1ley & Sons, Publ.

3/1940 Becket et a1. 148-31 I i 1 12/1941 Becket et a]. 14831 10 L. DEWAYNE RUTLED-GE, Pnmar'y Exammer 11/1961, Attenbmger et 1 75 123 W. W. STALLAIRD, Asslstant Exammer 9/1963 Tisdale 148-12 8/196-7 Schrader et a1 1482 2/1970 Goda et a1. 14s-12 2/1970 Shimizu et a1 148-12 15 6/1966 Shimmin et a1 75128 

